1. Field of the Invention
The invention relates to a hydraulic power steering system.
2. Discussion of Background
There is a conventional hydraulic power steering system that generates steering assist force by supplying hydraulic fluid from a hydraulic pump via a hydraulic control valve to a power cylinder coupled to a steering mechanism of a vehicle. In a common hydraulic power steering system, a hydraulic control valve is mechanically coupled to a steering member, such as a steering wheel, via a steering shaft, and the opening degree of the hydraulic control valve is adjusted on the basis of an operation of the steering member.
Japanese Patent Application Publication No. 2006-306239 (JP 2006-306239 A) describes a hydraulic power steering system that controls the opening degree of a hydraulic control valve with the use of an electric motor (valve driving motor) without mechanically coupling the hydraulic control valve to a steering member.
In the hydraulic power steering system in which the opening degree of the hydraulic control valve is controlled with the use of the valve driving motor, feedback control is executed on an electric motor (pump driving motor) for driving a hydraulic pump such that the rotation speed of the pump driving motor becomes a commanded pump rotation speed. However, the rotation speed of the pump driving motor may fluctuate due to fluctuations in load, or the like. For example, when a rotor of the hydraulic control valve is returned from an end position, at which hydraulic fluid supplied to one of cylinder chambers of a power cylinder becomes the maximum, toward a neutral position, the rotation speed of the pump driving motor fluctuates.
In this way, when the rotation speed of the pump driving motor fluctuates, steering assist force varies although the opening degree of the hydraulic control valve remains unchanged. Therefore, a steering feel may deteriorate.